Interdigital delay line



June 2, 1959 P. R. GUEINAQRD ET AL 2,889,486

' INTERDIGITAL DELAY LINE Filed March 19. 1953 2 Sheets-Sheet 1 June 2, 1959 P. R. GUENARD ET AL INTERDIGI'IAL DELAY LINE Filed March 19, 1953 2 Sheets-Sheet 2 2,889,48 6 Patented June 2, 1959 INTERDIGITAL DELAY LINE Pierre R. Guenard and Pierre C. Palluel, Paris, France, assignors to Compagnie Generale de Telegraphie Sans -Fil, a corporation of France Application March 19, 1953, Serial No. 343,316 Claims priority, application France April 3, 1952 3 Claims. (Cl. 315-35) The present invention relates to travelling wave tubes, and more particularly to the delay lines thereof.

Travelling wave tubes are known, with or without a transverse magnetic field, some of which operate as amplifiers and others as oscillators, and the delay line of which has an interdigital structure.

In its'usual form, such a delay line comprises two parallel bars supporting straight fingers which interengage when the line is assembled. The plane system thus formed is closed by metal shoes or plates which connect the bars with or without electrical contact therewith.

The known advantages of such a line include high heat dissipation which permits of increasing the rated power of the tube, high-grade properties with regard to the robustness and accuracy of the construction, the possibility of choosing appropriate dimensions for regu lating the form of the dispersion curve (that is to say, the value of the delay as a function of the wave-length and of the velocity of the beam), and the property possessed by a component of the field having a considerable amplitude, namely a negative phase velocity (propagation of the field inverse in relation to the propagation of energy), by reason of which this line can be successfully employed in travelling wave oscillators having a wide tuning band, in which the feed-back is established through the beam, as is explained in detail in copending United States application Serial No. 281,347 filed on April 9, 1952, in the name of Epsztein.

However, when such a line is employed in a travelling wave tube having no transverse magnetic field, in which the. beam is directed parallel to the line by axial electric field established between the cathode situated at one end of the line and a collector situated at the other end of the line, and in which an appropriate focussing device directs the larger part of the beam onto the collector, which may be cooled, the coupling between the line and the beam is relatively low, because the beam may be deflected away from the line if the focussing is inaccurate.

The present invention has for its object to obviate this disadvantage by the provision, in a tube of the type specified, of an interdigital line having curved fingers enclosing a cylindrical space into which the beam passes and in which a high-frequency field obtains when the line is energised.

The-various possible embodiments and the various ways in which they may be produced, as well as other features of the invention will be more readily understood by reference to the accompanying drawings, in which:

Figures 1 and 2 are respectively a plan view and a cross-section through a known delay line;

Figures 3A, 3B and 3C are respectively an end view of Figure 3B, a perspective view and a cross-section on line 3C-3C in Figure 3B, of the general form of delay line according to the invention;

Figure 4 illustrates a semi-finished component which may be shaped into a line according to Figure 3;

Figures 5 to 10 are respectively an end view, an

axial section, a transverse and an axial section, a crosssection, and a side elevation of five modified forms of delay line according to the invention;

Figure 11 is an axial section through a complete travelling wave tube provided with a delay line according to the invention; and

Figure 12 is an axial section through a modified portion of the tube in Figure 11.

In the known delay line of Figures 1 and 2, two

parallel bars 1 and 2 support two series of straight fingers 3 and 4 in the space between side plates 5 and 6.

Figure 3 shows the general form of a line according to the invention which comprises a bar 7 supporting a series of incompletely closed loops 8, 9 on alternate sides thereof.

There are a large number of methods by which a line of this form may be produced. For example, it is possible to start from a bar comprising two lines of straight fingers attached thereto opposite one another in staggered formation (Figure 4). The fingers are first bent in annular form so as to interengage. In another example, the bar may support transversely and at regular intervals, identical rings which are split on alternate sides of the bar, at a point level therewith, either before or after having been secured in position. In a further example, a bar may be secured along a helix, the con volutions of which are split level with the bar on alternate sides thereof. The line may also be produced by directly machining a bar. In all cases, a product similar to that shown in Figure 3 is obtained.

It is not essential for the loops of the line to be circular, but any other form in which they surround the beam may be conceived, for example an oval, flattened or rectangular form surrounding a flat beam. The choice of such a form, as illustrated in Figure 5, is favourable to an increase in the coupling between the beam and the high-frequency field.

The required form for the dispersion characteristic may be obtained by giving the fingers an appropriate profile. In some cases, fingers having a simple uniform section (for example rectangular, triangular or circular). may be employed, and in other cases two series of fingers having different forms may be employed. For example, the line, the axial section of which is shown in Figure 6, corresponds to a series of round and triangular fingers. staggered with respect to one another. I

Figures 7 and 8 show in cross-section and in axial section respectively a line element in which the fingers 81 and 9 have extensions 10 parallel to the flanks of the bar 7. Such a system affords a certain independence between the length of the fingers and the dimensions of the Zone through which the beam passes, and consequently provides a means of varying the dispersion char acteristic and/or the properties of the interaction space.

The bar itself may be given a special profile, for example a T-shaped profile as shown in Figure 7. It may be formed of two parts, each comprising a series of fingers, which are finally assembled one against the other.

A longitudinal slot 11 formed in the bar 7 (Figure 9). with a depth substantially equal to M4, in which A designates the wave length in the free space, ensures decoupling between the opposite ends of the fingers of the two series. The said slot could be formed either in a bar made in one piece, or by assembling two separate bars along the plane of the chain line 12.

Figure 10 shows transverse slots 13 formed in the bar 7 between the fingers 8 and 9. The fingers of the line are thus charged by stubs. Slots shorter than )t/ 4 modify the dispersion curve, especially at the lower boundary of the wave band, while slots longer than M4 afiect the upper limit.

A line of the type described may be more simply constructed than a line of the type illustrated in Figures 1 and 2, the lateral; shoes or plates no longer being required'. However, it may be desirable to limit the space occupied by the'high-frequency field on either side of the fingers, in order to define the interaction space and/ or to influence the form of the dispersion characteristic. Thus, as seen in Figure 12, an axial internal conductor 24 may be introduced when a tubular beam 23 is employed. Outside the fingers, the line may be surrounded by a cylindrical conductor, which may be formed by the envelope of the metal tube, the said envelope being metallised or covered with a layer of aquadag, or by a metal cylinder disposed outside the tube, for example the support of a magnetic focussing device.

Figure 11 shows diagrammatically in axial section a travelling wave tube provided with a delay line according to the invention.

A metal envelope 14 closed at its ends by insulating members 15, 16 contains the usual elements of a travelling wave tu'be, namely an electron gun 17, the anode 18 of which is incorporated in the envelope, and a collector 19. Rigidly gripped between the gun and the collector is a bar 7' which is applied against the envelope and supports the curved fingers 8, 9. The assembly 7, 8, 9 constitutes a delay line based on the principles illustrated in Figures 3 to 10, in which the characteristic features illustrated in the said figures may be employed in combination. In cases where the tube is to operate as an amplifier, or as an oscillator having an external feedback channel, the outermost fingers of the line will be connected either to co-axial elements or to pick-ups 20 matched to wave guides and projecting through the insulating passages 2.1.' Naturally, it the tube operates as an oscillator having internal coupling by reflection on the delay line or through the beam, there will be only one such co-axial element or matched pick-up.

The beam is focussed by a normal magnetic coil 22- supplying an axial field, or by any equivalent means.

The arrangement described affords numerous advantages, notably as compared with travelling wave tubes in which a helical delay line is employed. The rigidity of the line itself due to its construction, and its rigid support between the gun and the collector and/ or on the envelope, ensures great robustness and accurate centering of the line along the axis of the beam. The use of dielectric supports in which a certain quantity of energy is lost and which introduce disturbances into the inter action mechanism, as in the caseof the helix, is avoided. The natural evacuation of heat is very good, and is further improved by the contact between the outer face of the bar and the envelope of the tube. Moreover, the cooling may be intensified by a circulation of air or water in the bar, without affecting the high-frequency circuit. In addition, the dimensions'ot the cooling ducts are not limited by the dimensions of the fingers. None of these advantages can be obtained in the case of a helix;

' The tubes constructed in accordance with the invention may have a rated power far exceeding that permitted with helices. Furthermore, the beam may completely surround the line without the heat which results from the electron bombardment of the line prohibiting the operation of the tube. Such a tube is valuable, for

example, at very short wavelengths, in which the coupling between the beam and the line is stronger it the beam completely surrounds the line. If required, some of the fingers which undergo a greater dissipation, such as those in the neighbourhood of the gun, may be constructed of refractory metal. The correct matching of the line to the external circuits may be efiectedby individual fitting and adjustment of the outermost fingers, as well as by the choice of the connecting elements.

What we claim is: i

1. A delay line of interdigital structure defining a nearly enclosed interaction space for use in a travelling wave tube of the type having an electron beam adapted to propagate through said interact-ion space in energytransfer relationship with said delay line comprising a single longitudinal bar of substantial thickness and height and provided with opposite external faces, a plurality of fingers each; having two ends, said fingers being alternately mounted en said opposite faces of said. bar with one end therebt in interdigitated relationship with each other essentiallyover the entire length of said delay line, each of said fingers being curved to form with said bar a loop like structure defining said interaction space, and the other endof each of said fingers being spaced from the opposite face; of bar to. which the respective one end thereof is connected by a small distance to provide a gap, saidbar being provided with a longitudinal slot having a smaller; depth than sm'd height, said slot being open toward said interaction space.

2 A delay'line according to claim 1, wherein said bar is termed of two individual assembled elements.

3. A travelling wave tube comprising an elongated metalenyelope, a delay line of interdigital structure defitting a nearly enclosed interaction space including a single longitudinalbar of substantial thickness and height proyided was opposite external faces, a plurality. 'offingetseach having twoends, said fingers being alternately 'rno'nnted on said opposite faces of said bar with one end thereof in interdigitated relationship with each other essentially over the entire length of said delay. line, each of said fingers being curved to form with said bar a loop.- like structure defining said interaction space, the other end of each ot said fingers being spaced from the opposite face at said ban to which the respective one end thereof;

connected by a small distance to provide a gap, said hat beingproyided with a longitudinal slot having a smaller depth than said height, said slot being open toward, said interaction space, said bar extending longitudinally-of said envelope in contact with the inner wall thereof, means firmly gripping said bar against said envelope, means in: cluding an electron emissive source positioned relative to.

said delay line for emitting a beam of electrons at least a portion of which flows through said interaction space in energy transfer relationship with said delay line, and; at least 'one coupling means coupled to said delay line tb enable transmission of electromagnetic energy with re spect to said delay line. i 4

References Cited in the file of this patent UN ED. A ES. PAT NT 

